Which one of the following is not a conservative force?
- (a)Frictional force
- (b)Electric force
- (c)Gravitational force
- (d)Spring force
Correct — A, Frictional force. A conservative force is one for which the work done depends only on the starting and finishing positions, not on the path taken, and the total work done around any closed loop is zero (such forces also have a potential energy associated with them). Friction fails both tests: it always opposes the motion, it dissipates mechanical energy as heat, and the work it does grows with the length of the path travelled. It is therefore a non-conservative force.
- (b)Electric force — The electrostatic force is conservative — it has an associated electric potential energy and its work is path-independent.
- (c)Gravitational force — Gravity is the textbook example of a conservative force; the work it does depends only on the change in height, not the path.
- (d)Spring force — The restoring force of an ideal spring is conservative — it stores elastic potential energy and its work depends only on the displacement.
Forces are classed as conservative or non-conservative. A conservative force does work that depends only on the end points (not the path), does zero net work over a closed loop, and can be written in terms of a potential energy. Gravitational, electrostatic and ideal spring forces are conservative; friction and air resistance are non-conservative because they dissipate energy.
The distinguishing feature of friction is dissipation: it converts mechanical energy into heat and always opposes motion, so moving an object out and back does not recover the energy — the work depends on the total distance travelled. The three distractors are the standard conservative forces, each of which stores recoverable potential energy.
- A conservative force does path-independent work and zero net work over a closed loop.
- Conservative forces have an associated potential energy (gravitational, elastic, electrostatic).
- Friction is non-conservative — it dissipates mechanical energy as heat.
- The work done by friction increases with the length of the path taken.
- Assuming every everyday force is conservative; friction and air drag are not.
- Forgetting that a conservative force must do zero net work over a closed path.
Asked as which listed force is NOT conservative — identified by its energy-dissipating, path-dependent nature (friction).
No directly related past PYQ was found.
- practice — not a real PYQ
The work done by a conservative force on a body moving around a closed path is
- (a)always positive
- (b)always negative
- (c)zero
- (d)equal to the kinetic energy
Answer(c) zero — a conservative force does no net work over a closed loop.
- practice — not a real PYQ
Which one of the following forces dissipates mechanical energy as heat?
- (a)Gravitational force
- (b)Spring force
- (c)Frictional force
- (d)Electrostatic force
Answer(c) Frictional force — it is non-conservative and converts mechanical energy into heat.